1. Field of the Invention
The present invention generally relates to indoor digital data radio communication between a plurality of stations for the purpose of communicating among the stations and accessing various resources attached to the stations In the specifically disclosed environment, a plurality of mobile stations communicate with one or more fixed base stations attached to a computer system, such as a local area network. More particularly, the invention relates to a system in which the base station exercises control over access to the radio channel by periodically broadcasting messages that demarcate fixed intervals of time, called "frames". Furthermore, such messages subdivide frames in a variable manner so as to allow flexible allocation of time to three modes of use of the channel: base station-to-remote transmission, fixed time-slotted allocation of time to various remote stations, and an interval in which remote station may access the channel using various contention mechanisms.
2. Description of the Prior Art
It is very often the case that indoor data radio systems are installed for the purpose of permitting communications between mobile stations and applications and data residing in the computer system of a business enterprise. For example, the business enterprise may include a warehouse storing a diverse inventory. Mobile stations in the form of hand held computers and radio transceivers having bar code readers are used to check the quantities of inventoried products, and the data thus collected is transmitted to a base station for input to the computer system.
A typical structure has mobile stations communicating to a fixed station acting as a gateway or bridge between the radio environment and a conventional local area network (LAN). The fixed station relays messages to other LAN-attached resources. Communications are rarely directly between mobile stations; rather, messages are exchanged with applications residing in fixed nodes. In this structure, it is natural to add certain radio system management functions to those of the gateway, including coordination of the mobile stations, access to the common radio channel. This augmented gateway is referred to herein as a base station.
In indoor digital data radio systems, a key problem is providing efficient access to the channel, as that channel is limited in capacity and shared by all its users. Static channel allocation means, such as frequency division multiplex (FDM) or fixed time division multiplex (TDM), are inefficient for many forms of computer-to-computer traffic, known to be bursty in nature, while other services (e.g., speech transmission, control applications, etc.) require a fixed upper bound on maximum latency for channel access, best satisfied by periodic guaranteed access to the channel.
Efficient radio channel usage is the basic requirement for a practical indoor radio data network. The different types of traffic from various data terminals are usually bursty in nature and not predictable. Random access schemes are known for their short response time when channel load is light. When channel load increases, random access schemes become inefficient and may be unstable. On the other hand, controlled access schemes, such as polling, achieve much better channel use efficiency when loads are heavy. However, polling schemes suffer from overhead as the polling cycle has to be reduced to meet response time requirements.
A protocol for indoor digital data radio systems therefore should have the following characteristics:
1) short access time if the channel is lightly loaded,
2) good channel utilization if the channel is heavily loaded,
3) unconditionally stable,
4) simple to implement, and
5) matched well to typical traffic patterns, where most traffic is outbound from a base station attached to a LAN to the mobile stations.
One form of indoor data radio uses a transmission technique known as "spread spectrum", authorized by the U.S. Federal Communications Commission (FCC) in its regulations, part 15.247, for use in certain frequency bands without user license. Spread spectrum communications offer several advantages including low density power spectra and interference rejection. There are several types of spread spectrum systems including direct sequence digital systems, frequency hopping systems, time hopping systems, pulsed frequency modulated (or chirp) systems, and various hybrids. Of these, the direct sequence digital systems and the frequency hopping systems are perhaps the more widely implemented. In a direct sequence digital system, a fast pseudorandom code generator is used to modulate slower digital data which, in turn, modulates a carrier. In a frequency hopping system, a coherent local oscillator is made to jump from one frequency to another under the influence of a pseudorandom code generator.
The subject invention may be implemented using either direct sequence digital or frequency hopping types of spread spectrum communications systems. A description of these and other types of spread spectrum communications systems may be found, for example, in Spread Spectrum Systems, 2nd Ed., by Robert C. Dixon, John Wiley & Sons (1984), and Spread Spectrum Communications, Vol. II, by M. K. Simon et al., Computer Science Press (1985).